In an image forming apparatus that uses an electrophotographic process, an image is formed by developing a latent image borne on an image bearing member such as a photoconductor by using a toner to form a toner image and by transferring the toner image onto an image recording medium.
In a two-component developing method, there exists a problem that a magnetic carrier adheres onto the image bearing member. The magnetic carrier, adhered onto the image bearing member, is a possible cause of deterioration of the cleaning performance of a cleaning apparatus. Specifically, when the magnetic carrier adheres, because the magnetic carrier is pressed against the image bearing member at a transfer section to transfer a toner image, resulting in forming (causing) a raised area having a cratered shape around substantially the center of the section where the magnetic carrier is pressed onto the image bearing member. This raised area having a cratered shape damages the cleaning blade which constitutes a cleaning device and deteriorates the cleaning performance of the cleaning device. In other words, a toner slips beneath the cleaning blade and remains on the image bearing member as residual toner, forming streaky unevenness in the image which is formed in the subsequent image forming cycle.
As a countermeasure to the problem of magnetic carrier adherence, one means is to prevent the carrier adherence onto the image bearing member in the developing process, and another means is to recover the carrier which is adhered onto the image bearing member. Because the former means is known to be difficult to apply as the speed of image forming is increased, the latter means is widely applied.
Unexamined Japanese Patent Application Publication No. 1993-66678 (hereinafter referred to as Patent Document 1), Unexamined Japanese Patent Application Publication No. 1994-130820 (hereinafter referred to as Patent Document 2), Unexamined Japanese Patent Application Publication No. 1999-237788 (hereinafter referred to as Patent Document 3) and Japanese Patent No. 4010338 (hereinafter referred to as Patent Document 4) have disclosed apparatuses such that a magnet is disposed at such a position that is opposite to an image bearing member and the magnet removes, by magnetic attraction, the magnetic carrier which is adhered onto the image bearing member to recover the magnetic carrier. In the Patent Documents 1, 2, and 3, a roller which is rotatable and in which a roller-shaped magnet is installed is used and the magnet removes the magnetic carrier from the surface opposing the image bearing member. Then, recovery of the attracted magnetic carrier is attained by rotating the roller and separating the attracted magnetic carrier from a surface thereof, other than the surface which is opposite to the image bearing member.
In Patent Document 1, the separation of magnetic carrier attracted onto the roller is attained by a constitution wherein the magnet inside of the roller consists of a plurality of electromagnetic segments separated in the circumferential direction. In this constitution, it is configured such that at least one electromagnetic segment, which is not opposite to the image bearing member, is set to be inoperative while other electromagnetic segments are set to be operative so as to separate the attracted magnetic carrier from the electromagnetic segment which is not operative. In Patent Documents 2, 3 and 4, the magnetic carrier is separated by scraping the magnetic carrier from the surface of the roller with a scraper.
Patent Document 1 discloses a technique which is based on a constitution using a plurality of electromagnetic segments, resulting in the problem of increased complexity of control and apparatus, resulting in cost increase. Patent Documents 2 and 3 disclose techniques such that a scraper is contacted to the surface of the roller, resulting in occurrence of scratches on the surface of the roller by friction at each of such points of contact. Furthermore, when insulating toner is adhered to magnetic carrier, an insulating toner film is formed on the surface of the roller. Therefore, if it is configured so as to recover the magnetic carrier by creating an electric field between the roller and the image bearing member, the electronic field is altered by the toner film, resulting in degradation of carrier recovery performance, and thereby, a periodical maintenance and part replacement becomes necessary. Patent Document 2 also discloses a technique to recover the magnetic carrier by a recovery roller without using a scraper. However, elaborate arrangement of the magnetic pole of the recovery roller alone is insufficient to effect a high recovery of attracted carrier.